一直以来,碳材料以其优异的力学性能和优异的电化学双电层电容器(EDLC)性能,在生产柔性超级电容器电极方面显现出巨大的潜力[10⇓-12].在众多碳材料中,石墨烯基电极因高机械性能和电化学性能而备受关注[13-14].文献[15]报道了一种用于超级电容器的改性石墨烯,并分别测试了该电极在有机电解液和水系电解液中的比电容,分别可以达到99 F/g和135 F/g,表现出优异的电容性能[15].随后,大量研究表明石墨烯电极具有优异的电容性能和电化学稳定性[16⇓-18].事实上,石墨烯作为一种迷人的二维纳米材料,可以通过π-π共轭或氢键作用形成连续的宏观石墨烯薄膜、石墨烯纸、石墨烯气凝胶等[19-20].以石墨烯纳米片或氧化石墨烯纳米片为基材的柔性石墨烯纸已经通过流动自组装的方法制备出来.该石墨烯纸具有高达35 GPa的优异拉伸模量和7 200 S/m 的室温电导率[21].这些优异的特性使得石墨烯纸可以用作柔性超级电容器的独立和无粘合剂电极[22].
研究表明,石墨烯纸的电容来源于电极/电解质界面的双电层.由于EDLC的性质,石墨烯基柔性超级电容器的电容性能受到限制,将MnO2、SnO2、VO2、导电聚苯胺(PANI)、聚吡咯(PPy)等赝电容材料引入石墨烯中是进一步提高石墨烯纸类柔性超级电容器容量的最佳途径[23].同时,在干燥和热处理过程中,由于强大的层间范德华力,石墨烯薄膜会发生不可逆的片间重新堆积,限制了电解质离子的可获得性,从而在很大程度上阻碍了电极表面和活性中心的充分利用[24-25].为解决这一问题, B、N、P等杂原子可作为隔层分子.氮化硼(BN)也被称为“白色石墨”,其具有非常相似的结构和良好的介电性能,可以作为一种合适的分子.sp2键合的B、N原子具有许多优良的性质,如高机械强度、高导热系数、化学稳定性和机械稳定性,使其成为很好的纳米膜[26-27].显然,将石墨烯与BN纳米片结合,可以发挥BN与石墨烯的协同效应,制备出高性能的超级电容器活性材料.然而,已有的制备石墨烯与其他二维材料的复合方法较为复杂.而BN纳米片作为一种新的2D层状材料,可以在有机溶剂和合适的表面活性剂混合溶液中通过直接液相剥离(LPE)来制备.因此,BN纳米片/石墨烯复合纸可以通过简单的真空辅助过滤工艺制备,并用于柔性超级电容器中.
基于上述分析,本文制备了基于超大尺寸氧化石墨烯(ULRGO)纳米片和溶剂剥离BN纳米片的新型高性能柔性全固态超级电容器(FASS).在去离子水中直接超声处理氮化硼粉末制备了BN纳米粉体.然后,将BN纳米片水分散液与均匀的超大尺寸石墨烯(ULGO)水分散液混合,形成ULGO纳米片/BN用于制备复合纸.最后,通过真空辅助过滤和氢碘酸还原得到了ULRGO/BN纳米片状(U/B)纸,柔性U/B纸具有良好的机械性能和导电性.与全固态超级电容器器件配套使用,U/B纸器件具有较高的比电容和较长的循环寿命.在进行 5 000 次充放电后,FASS的最高面积比电容达到325.4 mF/cm2,并具有约86.2%的高容量保持率,这表明U/B纳米复合纸是一种有前途的柔性超级电容器电极材料.
1 实验
1.1 样品制备
ULGO的制备:以剥离石墨(45目,质量分数99%)为原料,采用改进的Hummers法制备ULGO.首先,将6.0 g剥离石墨粉在室温下用混合酸(180 mL H2SO4,60 mL HNO3)处理24 h,得到石墨层间化合物(GICs).其次,将干燥的GICs粉末在管式炉中于 1000 ℃ 氮气中热膨胀10 s,得到膨胀石墨(EG).随后,将5.0 g EG在300 mL H2SO4、4.2 g K2S2O8和6.2 g P2O5中进行预氧化,在 80 ℃ 下机械搅拌5 h.再将预氧化的EG粉末在550 mL H2SO4和25 g KMnO4混合物中在35 ℃下进一步氧化5 h,然后将预氧化的EG粉末在管式炉中以1000 ℃的氮气热膨胀10 s并在300 mL H2SO4、4.2 g K2S2O8和6.2 g P2O5中预氧化5 h.最后,分别用质量分数10%的盐酸和去离子水洗涤10次,除去残留的金属阳离子和酸,得到胶体分散的ULGO.
溶剂剥离BN纳米片:在去离子水中直接超声BN粉末得到BN纳米片.具体为将2.0 g原始BN粉末(99.9%金属基,颗粒直径为 2 μm)添加到 100 mL 去离子水中,然后,将BN/去离子分散体密封在玻璃瓶中,并用尖端超声仪(SCHENTZ-650E,500 W)进行超声.在恒温水浴中超声10 h后,以 10000 r/min 的转速离心30 min,分离出未剥落的BN颗粒或厚片BN.取上层清液,得到剥离的BN纳米片/去离子水分散体.
U/B复合纸的制备:ULGO/BN复合纸的制备采用真空辅助过滤的自立式组装方法.首先,将所需量的超细BN纳米片/去离子水分散体(BN纳米片/去离子水分散体的质量浓度为5 mg/mL)先用浴液超声剥离2 h.然后,缓慢加入剥离后的ULGO水分散液中.对混合物分散体进行超声洗浴2 h,并进行真空辅助过滤.通过聚偏氟乙烯(直径47 mm,孔径 0.22 μm)薄膜过滤,乙醇洗涤3次后风干,制备了不同进料质量比的ULGO/BN独立式复合纸.剥离复合纸后,用质量分数55.0%~58.0%的氢碘酸在80 ℃下浸泡2 h还原得到ULRGO/BN复合纸,分别用饱和碳酸钠水溶液和去离子水洗涤得到U/B复合纸.最后,将U/B复合纸放置在两个玻璃板之间,并放在80 ℃真空干燥72 h.
1.2 仪器与表征
用Bruker D8 Advance X射线衍射仪、Thermo Scientific Escalab 250 Xi X射线光电子能谱、JEM-2100F透射电镜、 原子力显微镜(AFM)、JSM-7800F场发射扫描电子显微镜(SEM)以及LabRAM HR 800共聚焦拉曼光谱进行表征.复合纸的电导率采用四探针法,在装有平行探针(探针间距为1 mm)的样品平台下测定.在温度约30 ℃、湿度约25%、十字头速度为2 mm/min的条件下,用CMT-4000机测试复合纸的拉伸性能.
1.3 电化学测量
使用CHI660E电化学工作站(中国上海辰华仪器有限公司)通过循环伏安、恒流充放电和电化学阻抗谱对电化学性能进行评价,采用传统的三电极系统:工作电极为U/B复合纸,对电极为铂板,参比电极为饱和甘汞电极.水溶液电解质为1 mol H2SO4.在60 mL去离子水中混合6 g H2SO4和6 g聚乙烯醇(PVA)制备PVA/H2SO4凝胶电解质,在剧烈搅拌下持续加热至85 ℃,保温1 h.在全固态超级电容器(SC)的情况下,用PVA/H2SO4电解质的热透明溶液涂覆两张长、宽分别为2、 0.5 cm的U/B复合纸,并在室温下风干以蒸发多余的水分.将这2个电极压在一起,形成一个集成的FASS器件.电压范围为0~1 V,电压扫描速率为5~100 mV/s,电流密度为0.25~5 mA/cm2,交流阻抗测试范围为0.1~100 kHz,幅度为5 mV,参考开路电位.
2 案例分析
图1所示为U/B复合纸制备过程.用去离子水超声处理BN粉末制备BN纳米片[28],如图1(a)所示.用尖端声化器(SCHENTZ-650E,300 W)超声10 h后,在 10000 r/min转速下离心分离30 min,得到BN纳米片/去离子分散体.用改进的Hummers方法从膨胀石墨中制备纳米片[29].从图1(b)和1(c)可以看出,得到的ULGO纳米片/BN水分散体是均匀和稳定的.此外,以膨胀石墨为原料,采用改进的Hummers法制备了ULGO纳米片,得到的ULGO水分散体和ULGO与BN混合液是均匀、稳定的.ULGO/BN复合纸通过对ULRGO和BN纳米片混合分散的吸滤而组装,如图1(d)所示.
图1
图2
分散在溶液中的氧化石墨烯可以在定向流动下组装成有序的结构,从而得到氧化石墨烯基纸.与石墨烯类似,具有石墨烯结构的二维BN纳米片也可以在定向流动作用下重新堆积,形成连续的宏观结构.令ULGO和BN的质量比为δ,通过混合所需体积的ULGO和BN纳米片的胶体悬浮液,进行真空辅助自组装以共堆叠U/B复合纸,制备δ=1∶1,1∶2,1∶3的ULGO/BN复合纸.然后,用氢碘酸将ULGO/BN纸化学还原成U/B纸,以去除氧基,恢复其电学性能,并提高其强度.可以将获得的U/B纸弯曲到 180 °,不会造成任何破坏,如图3(a)和3(b)所示.利用SEM对U/B复合纸进行表征,如图3(c)~3(e)所示,图3(c)和3(d)显示U/B复合纸的断口在整个横截面上呈现明显的层状结构,类似于用相同方法制备的石墨烯基纸的微观结构[30].同时通过SEM能谱仪相对(EDS)图谱分析结果可以看出,C、O、B和N元素均匀分布在材料表面(见表1,表中w1为元素在所有元素中的质量分数,w2为元素在C、O、N、B中的质量分数,a为元素相对原子质量分数).剥离的BN纳米片和ULGO纳米片共堆积成独立的珍珠层状U/B复合纸.与ULGO纳米片相比,剥离的BN纳米片的横向尺寸相对较小.因此,U/B复合纸断口的SEM照片上没有明显的BN纳米薄片.事实上,在共堆积之后,小的BN纳米薄片被插入到复合纸的石墨烯层之间.通过EDS分析,C、O、B和N元素均匀分布在局部表面(见图3(e)).B和N元素来源于剥离的BN纳米薄片,这表明剥离的BN纳米薄片成功插入到复合纸的层状结构中[31].由于剥离的BN纳米片和ULGO纳米片可以组装成宏观柔性纸,所以将不同质量比(1∶3、1∶2 和1∶1)的U/B复合纸分别定义为U/B-3、U/B-2和U/B-1复合纸.
图3
图3
U/B纸弯曲到 180 ° 结构未破坏的图像
Fig.3
Images of U/B paper bending to 180 ° without damages
表1 U/B复合纸的表面的EDS数据
Tab.1
| 元素 | 线类型 | w1 | w2 | 质量分数 标准误差 | a | 标准样品 标签 |
|---|---|---|---|---|---|---|
| C | K线系 | 48.42 | 56.97 | 0.15 | 63.95 | 纯元素 |
| O | K线系 | 15.19 | 42.64 | 0.15 | 35.93 | SiO2 |
| N | K线系 | 0.30 | 0.28 | 0.01 | 0.12 | 纯元素 |
| B | M线系 | 0.12 | 0.12 | 0.04 | 0.01 | 纯元素 |
| 总量 | 100.00 | 100.00 |
由于独特微观结构源于导电ULRGO纳米片和BN二维材料两者相结合,所以ULRGO/BN混合薄膜具有显著提高超级电容器电极电化学性能的潜力.柔性超级电容器的电化学性能采用1 mol/L KOH电解液进行循环伏安法测量.使用三电极配置,将独立的薄膜作为工作电极,铂箔(长、宽均为 1 cm)和饱和甘汞电极(SCE)分别作为对电极和参考电极.测试中未使用金属支架或集流体,薄膜直接固定在电极支架上进行连接.
图4(a)所示为ULRGO、BN 和 ULRGO/BN 混合纸张的循环伏安(CV)曲线,扫描速率高达 10 mV/s,在-0.2~0.8 V 电压窗口内.可以看出,ULRGO薄膜的CV曲线呈近长方形,没有明显的峰值,显示出碳基材料的电动双层电容特性.对于纯BN,CV曲线也表现出明显的双电层的电化学特性.图4(b)为不同的扫描速度下 δ=1∶1的CV曲线.阴极峰值右移至高压,阳极峰以更高的扫描速率转移到低值.显然,峰值电流密度随着扫描率的增加而增加.根据不同扫描速率下CV曲线周围区域计算出的理论电容没有明显差别,表明其具有良好的速率能力.此外,对ULRGO/BN复合薄膜进行了恒流重放电(GCD)测量,其电流密度为0.5 mA/cm2,如图4(c)所示.所有曲线都表现出近似对称的三角形形状,揭示了ULRGO/BN复合薄膜的双电层电容行为.注意到不同δ值的充电放电曲线转折点上,阻抗消耗的电动势(IR)明显下降约 0.6~0.8 V,这表明引入BN产生了高内部和等效串联电阻(ESR),从而造成能量损失.电极材料的面积比电容根据下式计算:面积比电容=放电电流×放电时间/(电压变化×薄膜表面积),即Ca=IΔt/(ΔVS),其中I为放电电流,Δt为放电时间,ΔV为电压变化,不包括放电过程中的欧姆极化下降,S为薄膜的表面积.纯ULRGO和纯BN的比电容约为79.6和65.4 mF/cm2.ULRGO/BN复合薄膜材料的Ca明显高于纯ULRGO和BN.当ULRGO纳米片和剥落的BN 纳米片的质量比为1∶2,其显示的面积比电容为±223.2 mF/cm2,并表现出最佳电容性能.电容性能的提高表明,ULRGO纳米片和剥落的BN纳米片之间具有协同效应.图4(d)显示δ=1∶2时不同电流密度的电荷放电曲线,范围为0.25~5 mA/cm2.在 0.2 ~ 0.8 V的潜在范围内,曲线形状几乎没有变化,这显示了ULRGO/BN复合薄膜在较宽的电流范围内具有可持续性.根据ULRGO的比容量,在图4(e)中绘制了不同电流密度下ULRGO、BN以及δ= 1∶1时的比电容对比图,以便进行清晰比较.对于所有薄膜,随着电流密度的增加,比电容减少.对于δ= 1∶1的复合薄膜材料,比电容从0.25 mA/cm2的±252.3 mF/cm2下降到 1 mA/cm2 的220.28 mF/cm2,电容保留率为87.31%.即使在5 mF/cm2时,δ= 1∶2薄膜的比电容仍高达±145.1 mF/cm2.然而,当在电流密度为0.25及0.5 mA/cm2时,纯BN薄膜的比电容从79.5 mF/cm2下降到50.2 mF/cm2,容量保留率仅为64.3%.当电流密度增加到 5 mA/cm2,比电容保留率仅为约32.8%.结果表明,ULRGO/BN复合薄膜在高电流和高速率性能方面比纯BN薄膜具有更好的可持续性.此外,在电流密度为0.5 mA/cm2下测试了δ=1∶2的ULRGO/BN复合薄膜的循环性能.可以清楚地观察到,电容的显著减少只发生在500次循环之前.经过 5000 次循环后,δ=1∶2的 ULRGO/BN复合薄膜的容量保留率仍为 87.31%,高于ULRGO的容量保留率76.32%.一般而言,由于ULRGO纳米片和BN纳米片之间的部分接触无效,导致电子转移和离子扩散的恶化,从而导致比电容的损失.ULRGO作为一种极具吸引力的碳材料,由于其电导率和结构稳定性, 通常表现出良好的循环稳定性.加入剥离的BN纳米片后,其循环性能得到了改善.这主要是由于ULRGO纳米片与BN纳米片之间的协同效应.
图4
图4
柔性超级电容器的电化学性能测量
Fig.4
Electrochemical performance measurement of flexible supercapacitor
为进一步证明复合薄膜的优点,选择在0.1~100 kHz 的频率范围内对ULRGO/BN复合薄膜的电化学阻抗谱进行测试,如图5所示.图中:rGO为氧化石墨烯,交流振幅5 mV,Z″ 为实验处理中得到的虚部.从图5(a)中的电化学阻抗光谱来看,实部(Z')的拦截是电解质的离子电阻、基板的本征电阻、活性材料的本征电阻抗以及活性材料/集流体的接触电阻的组合.电极的电荷转移电阻(Rct)从高频区域半圆的直径计算,而低频的直线则呈现了离子的扩散行为.斜线的更陡峭形状代表一种理想的电容行为,表明电解质中离子的扩散速度更快.从放大高频区域的半圆直径来看,ULRGO/BN复合薄膜的电阻高于纯ULRGO薄膜.电阻增加的主要原因是BN薄膜的导电性差.从另一个方面来说,ULRGO的引入明显提高了复合薄膜的导电性.图5(b)为不同质量比的ULRGO/BN复合纸的EIS谱图.从较低频率的直线上看,δ=1∶1和1∶2薄膜的斜率高于纯ULRGO和BN薄膜.结果表明,ULRGO/BN复合薄膜具有良好的导电性和离子扩散性能,是其作为超级电容器电极材料具有良好性能的主要原因.
图5
图5
ULRGO/BN复合薄膜的电化学阻抗谱
Fig.5
Electrochemical impedance spectrum of ULRGO/BN composite film
图6
使用循环伏安法测试了作为制造的FASS的电化学行为.图6(a)显示以不同扫描速率测试的δ=1∶2复合薄膜组装的器件的CV曲线.与三电极系统不同,δ=1∶2复合薄膜基FASS的CV曲线在双电极系统中没有明显的氧化还原峰值,扫描速率不同的近乎理想的矩形形状显示了柔性薄膜的极佳电容特性和超快响应.从图6(b)看出,δ=1∶2复合薄膜的FASS的GCD曲线是理想的三角形形状.在0.25 mA/cm2的电流密度下,基于δ=1∶2复合薄膜的FASS的比面积电容约为325.4 mF/cm2.随着电流密度增加到 5 mF/cm2,面积比电容也保持在 ±230.2 mF/cm2,显示了FASS器件的主导速率能力.从图6(c)来看,从基于不同质量比的ULRGO/BN复合薄膜的FASS器件电流密度和面积比电容之间的关系来看,δ=1∶2复合薄膜基FASS器件的电容性能明显优于其他质量比的薄膜基FASS器件,与先前3次电极测试的分析数据是一致的.
为了验证ULRGO/BN复合薄膜组装FASS器件的柔韧性,在50 mV/s的扫描速度下弯曲器件测试循环伏安曲线.此外,从图6(d)中可以看出,FASS器件在 5000 次循环后的高循环稳定性表现为约86.2%的高容量保留.从图6(e)来看,在不同的弯折角度下,ULRGO/BN薄膜组装的FASS的CV曲线之间几乎没有任何变化,表明ULRGO/BN复合薄膜对称FASS器件具有很高的柔韧性.不同电流密度下的FASS器件的面功率和能量密度如图6(c)所示.FASS 的能量密度为22.8 W·h/kg,功率密度为85.7 W/kg.而当功率密度增加到 2 094.7 W/kg,在0.8 V的工作电压下,还保留了71.2%的能量密度,优于先前报告的对称器件,如图6(f)所示.为了证明基于ULRGO/BN复合薄膜组装的FASS器件应用潜力,将具有相同负载(每个电极约为2.0 mg)的组ULRGO/BN复合薄膜装超级电容FASS器件串联起来,形成串联装置.1个1.5 V、3 W的红色发光二极管(LED),使用3个系列器件(见图6(e))可以点亮25 s以上,显示了该固态超级电容作为可穿戴的柔性超级电容器件的应用潜力.
3 结论
基于ULRGO和剥离BN纳米片制备柔性导电U/B复合纸.通过真空辅助过滤ULGO/BN纳米片的混合液和还原碘化氢,成功地制备了镶嵌在ULRGO层间的BN纳米片的珍珠层状U/B复合纸.该U/B复合纸可用作FASS电极,具有比电容高、倍率性能好、循环性能好等显著的电化学性能.
(1) ULRGO纳米片之间良好的接触形成了高电导率和高孔隙率的互连结构,有利于电子转移和电解质离子扩散.
(2) 相对较小的BN纳米片层分散均匀,并插入到ULRGO片层之间.因此,它既保持了U/B复合纸在循环过程中的结构稳定性,又充分利用了伪电容BN纳米片进行电荷存储.
(3) 与石墨烯/过渡金属氧化物复合材料类似,电活性BN与导电ULRGO之间的协同作用对提高材料的电化学性能起着重要作用.
BN纳米片和超大尺寸石墨烯纳米片复合形成超级电容在储能方面具有广阔的应用前景,同时依据优越的电子转移和电解质离子扩散性能,利用高孔隙率结合石墨烯与其他材料如锌钨混合分子、碳纤维、NX04H等构成聚合物薄膜再形成双芯电力电容器,能达到通过电压调节容量输出的效果,对低压无功补偿领域有较强的指导意义.
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